Soret-Effect Induced Phase-Change in a Chromium Nitride Semiconductor Film

Phase-change materials such as Ge–Sb–Te (GST) exhibiting amorphous and crystalline phases can be used for phase-change random-access memory (PCRAM). GST-based PCRAM has been applied as a storage-class memory; however, its relatively low ON/OFF ratio and the large Joule heating energy required for the RESET process (amorphization) significantly limit the storage density. This study proposes a phase-change nitride, CrN, with a much wider programming window (ON/OFF ratio more than 105) and lower RESET energy (one order of magnitude reduction from GST). High-resolution transmission electron microscopy revealed a phase-change from the low-resistance cubic CrN phase into the highly resistive hexagonal CrN2 phase induced by the Soret-effect. The proposed phase-change nitride could greatly expand the scope of conventional phase-change chalcogenides and offer a strategy for the next-generation of PCRAM, enabling a large ON/OFF ratio (∼105), low switching energy (∼100 pJ), and fast operation (∼30 ns).

It consisted of a square W plug (heater electrode) with a side length varying from 34 to 218 nm.The fabrication process was initiated from patterning and photolithography.(Step-1) In this Step-1, a negative photoresist (ZPN1150-90, ZEON, Japan) was spincoated onto the substrate using a spin-coater (ACT-300A, ACTIVE) at a speed of 4000 rpm and then baked at 105 ºC for 120 s after coating.The device was then patterned using a photolithography machine (PEM-800, Union Optical Co., Ltd.).The second bake process was carried out at 115 ºC for 60 s immediately after photolithography.
The device was developed in the developer (NMD-3, TOKYO OHKA KOGYO CO., LTD.) for 70 s and cleaned with distilled water for 50 s.The remaining resistor in the patterned area was cleaned using a UV irradiation chamber for 180 s.In Step-2, the device was first reverse sputtered for 73 min to remove the surface oxidation of W-plug and the CrN or CrN' layer (100 nm) and the W top electrode layer (200 nm) were sequentially deposited onto the patterned area after photolithography at room temperature by sputtering.In Step-3, a lift-off process was conducted by immersing the device in acetone solvent at room temperature until all the W/CrN/resist layer was removed.The device was finally cleaned with ethanol (60 s), distilled water (60 s), dried and wait for test.The CrN device was fabricated with the plug's side length of 37, 45, 81, 113 and 218 nm and the CrN' device with the size of 34, 37, and 45 nm.10.Crystal structure of the CrN' thin film.
The CrN' film was deposited using the same sputter chamber and conditions except for the working pressure, which was 1.3 × 10 −1 Pa.The TEM and XRD of CrN' film were measured using same method with CrN thin film.We tested the switching performance of the CrN'-based memory device under electrical pulses.Fig. S11a displays the resulting RV characteristics.For a device with a plug size of 34 nm × 34 nm, when we applied a positive voltage pulse to the bottom electrode (BE), the device reached the HRS of ~10 8 Ω at 1.2 V and then, recovered its initial LRS reversibly upon application of a higher voltage (1.5 V).Devices with larger plugs exhibited similar RV trends, indicating nonvolatile and reversible resistive switching, i.e., LRS-to-HRS (RESET process) and HRS-to-LRS (SET process).The highest HRS resistance observed was ~10 9 Ω, and the HRS/LRS resistance ratio was as high as 10 6 .
When a positive-current pulse sweep (pulse width: 500 µS) is applied to the HRS, typical threshold switching was observed, similar to that observed in a CrN memory cell.(Fig. S11b) that is enclosed by a light-blue square in Fig. S11d; the bright and darker spots were supposed to be Cr and N atoms, respectively.On the matrix side, the IFFT image shows a clear cubic atomic column structure of CrN', which is consistent with the lattice parameter a = 4.17 Å of a NaCl-like cubic structure.For the active region, the IFFT image reveals a new atomic column structure that can be indexed not as cubic but as hexagonal, indicating a phase change from cubic to a hexagonal structure.12.The carrier types in CrN and CrN' thin films.

Febvrier et al. recently obtained p-type
CrN by controlling the stoichiometry without the introduction of additional dopants using DC magnetron sputtering; they demonstrated that the p-type conduction can be attributed to Cr vacancies, which push the Fermi level down toward the valence band. 8In the present study, we first obtained a CrN film with a composition of Cr 47.5 at%, N 43.9 at%, and O 8.6 at%, which was detected by RBS, where the Cr/(N+O) composition ratio was 0.91.A small amount of oxygen was introduced in the case of sputtering deposition at a high working pressure.
In this case, the total concentration of N and O anions exceeded that of Cr, indicating a

EXAFS measurements on CrN
We directly observed the chemical environment of Cr k-edge in our CrN thin film by Extended X-ray Absorption Fine Structure (EXAFS, beamline BL01B1@SPring-8).In   The band structure was studied to analyze the origin of semiconductor type differences.
The absorption coefficient α was calculated from the following equation: 11 Where  is film thickness,  is transmittance, and R is reflectance.The bandgap E g can be estimated from the α vs. wavelength curve using the Tauc plot method: 11 Where ℎ is Planck constant,  is frequency, and  is a proportional constant.The value n, which equals 2 or 1/2, can determine the indirect and direct transition, respectively.
The reflectance and transmittance spectra of CrN and CrN' films are shown in Fig. S14a and b.Fig. S14c shows the (αhv) 1/2 as a function of ℎ for CrN and CrN' thin films with a thickness around 100 nm, which exhibits an indirect transition in the Tauc plot.By extrapolating the linear region to abscissa yields, the bandgap was determined to be 0.18 eV for CrN' and 0.77 eV for CrN thin films, respectively.To understand the position of Fermi level in the band structure, the valence band spectra of the CrN and CrN' thin films were measured by HAXPES, as shown in Fig. S14d and e.The Fermi level relative to the valence band maximum (VBM) (E F -E v ) was estimated to be 0.05 and 0.1 eV for CrN and CrN' thin films, respectively.The schematic of the band structures was depicted in the inset of Fig. S14d and e.In the CrN thin film, the fermi level locates near the valence band, indicating p-type conduction.While in CrN', the fermi level is closer to the conduction band, which is a typical n-type semiconductor.
From the band structure, the intrinsic reason for different semiconducting types can be understood.To understand the driving force of ion diffusion and phase change in CrN-based memory cells, the thermal and electrical physics of the memory devices were simply modeled without considering the thermoelectric effect (Thomson and Peltier effects), thermal boundary resistance, and electrical interface resistance.The CrN/metal electrode interfacial properties could also be ignored because the contact resistance has been confirmed to be a minor factor in the device operation and CrN behaves more like a metal than a semiconductor in conducting property terms.A constant voltage of 0.2 V was driven as a simulation input from the BE, and Joule heating was used to determine the heat generation throughout the structure.The simulations were based on the current conservation law and energy conservation law as follows: and where  is the electrical conductivity,  is the current density, ∅ is the electrostatic potential,  is the density,   is the specific heat, T is the temperature, t is the time, k is the thermal conductivity, and   is the resistivity.The program that solves these equations is implemented in the open-source software, OpenFOAM.The corresponding values for CrN, TiN, W, and SiO 2 in the devices used for the calculation are listed in Table S2.

Thermal stability of CrN 2 phase.
We measured the temperature dependence resistance change (R-T) of the memory device.Since the pad size of T-shape device is too small for the probes size in our probe-furnace system, we fabricated the new structure device with larger electrode pads (see the schematic device structure in the inset of Fig. S16a, and the fabrication flow in ref 17 ).The memory cell was initially in a low resistance Set state and was Reset to a HRS (5 × 10 7 Ω) by applying voltage pulses as shown in Fig. S16a.Note that the smaller resistance contrast in this device is possibly due to the larger contact area (3×10 4 nm 2 ) between CrN and W electrode.The memory cell in high resistance Reset state was then transferred to the probe furnace.The furnace was firstly vacuumed to 10 -1 Pa and the film was then annealed in a gradient temperature with a heating rate of 10 °C/min up to 400 °C under the Ar atmosphere.Fig. S16b shows R-T curve of the HRS CrN memory cell.The resistance decreased slightly with increasing temperature until the phase transition point (T phase change ) was reached, after which the resistance decreased sharply corresponding to a phase transition from CrN 2 to CrN.The T phase change was then determined to lie within the range of 250°C to 300°C by taking the minimum of the first derivative of the R-T curve, which is much larger than the crystallization temperature of traditional PCM: GST (~150 ºC). 18Another CrN memory cell was also evaluated for the thermal stability as shown in Fig. S16c using the same method.The T phase change was found to be located in the range of 250°C to 300°C, emphasizing the thermal stability of the CrN 2 phase.

Cyclic resistive switching of CrN memory.
We investigated the feasibility of cyclic resistive switching in both CrN-based memory devices (Fig. S17a).Although our devices exhibited resistive switching under unipolar positive voltage pulses, the narrow window of the pulse voltage applied for switching to the HRS (as small as 0.2 V) made it difficult to switch repeatedly under a constant pulse width.For example, the HRS was sometimes hard to achieve accurately because a little bit higher voltage could induce an LRS beyond the voltage limit for the HRS.In T-shaped PCRAM operation, positive electrical pulses are generally applied to the TE for both the RESET and SET processes because of the higher thermal efficiency compared with the case of negative electrical pulses applied to the BE 19 .Therefore, to investigate the endurance of the CrN-based memory devices, we adopted a negative electrical pulse direction to lower the thermal efficiency for the SET process, which enabled a large window of pulse voltage for the RESET/SET switch.The CrN-based memory device with a plug size of 45 nm × 45 nm showed bipolar switching.The device with an LSR of ~2 × 10 4  started to switch to an HRS of ~2 × 10 8  when applying a positive voltage pulse of 1.4 V, and it went back to the LRS after applying a negative voltage pulse of −1.4 V with a width of 50 ns (Fig. S17b).
Note that the programming window in the bipolar cyclic measurements was about 10 4 , still limited compared with other PCMs such as GST.Thus, we investigated the failure mechanism of the CrN memory device for better cyclability.Fig. S17c-e shows the Reset/Set voltage dependence of the endurance properties.The endurance was tested using the same cell size (45 × 45 nm 2 ) with fixed pulse width 50 ns and read voltage 0.1 V.As shown in Fig. S17c, when the Set and Reset voltage were set to -2 V and 2 V, respectively, the memory cell could be cycled for more than 20 times with a HRS/LRS resistance contrast of around 10 5 .When the Set and Reset voltages were reduced, the resistance contrast between HRS and LRS decreased to around 10 4 , but with an improved cyclability of 10 3 (Fig. S17d).To improve endurance, various approaches have been proposed, such as engineering the bottom electrode 21 or utilizing a confined memory structure with a thin metallic liner to restrain atomic displacement 22 .However, the HRS/LRS resistance contrast would also be inevitably degraded with more cycles (e.g., On/off < 10; Cycle > 10 12 for GST in a confined PCRAM). 6,23Therefore, we believe that much better cyclic property can be obtained in the same confined PCRAM structure, and it will also be interesting to investigate the trade-off relationship of the on/off ratio and endurance in the new structure in our future work.

1 .
Fig. S1.Process flow to fabricate CrN layer and top electrode: T-shaped memory cell substrates cut from substrate wafer with the W-plug electrode were used in this study.

Fig. S2 .
Fig. S2.(a) Bright-field cross-section TEM micrograph, revealing a columnar-like grain microstructure and a relatively smooth film surface; the inset displays a selected area electron diffraction (SAED) pattern of the film, showing a ring pattern of the polycrystalline phase that can be attributed to a single NaCl-like cubic CrN phase.(b)In-and out-of-plane XRD spectra at room temperature, indicating a cubic phase.

Fig. S7 . 8 .
Fig. S7.(a) FFT and (b) IFFT image derived from the [121] zone axis, indicating the location of N atoms.(c) Simulated thickness/defocus map of (b) obtained by the multislice method from HRTEM images by using the QSTEM software 7 ; for the simulation, the lattice parameters  = 2.71931 Å and  = 3.71155 Å were used for hexagonal CrN 2 .The IFFT images were obtained using the Gatan DigitalMicrograph software.

Fig. S10 .
Fig. S10.(a) Bright-field cross-section TEM micrograph, exhibiting a microstructure similar to the CrN thin film shown in Fig. S2a, along with a SAED pattern (inset) showing a ring pattern of the polycrystalline phase, which can be indexed as a single NaCl-like cubic CrN phase.(b) In-and out-of-plane XRD spectra at room temperature, indicating a cubic phase.
observations were conducted to investigate the resistive switching mechanism of the device with a plug size of 218 nm × 218 nm.Before the TEM analysis, the device sample was switched to an HRS of ~106 Ω by applying a voltage pulse of 5 V for 50 ns.A distinct bright contrast was observed in the CrN' layer between the W plug and TE electrode (Fig.S11c).Fig.S11ddisplays a high-resolution TEM (HRTEM) image taken at the boundary region between the matrix (upper part) and active region (lower part) indicated by a light-blue square in Fig. S11d.Fig. S11e illustrates an inverse fast Fourier transform (IFFT) image of the boundary region between matrix and active areas

Fig. S11 .
Fig. S11.(a) Resistance as a function of the pulse voltage for various plug sizes; the pulse width was fixed at 30 ns and the read voltage was 0.1 V. (b) Threshold switching behavior, showing a resistance change from ~10 8 (high-resistance state: HRS) to ~10 3 (low-resistance state: LRS).(c) Cross-sectional transmission electron microscopy (TEM) image of the device with a 218 nm × 218 nm plug, which was previously reset to a high-resistance state.(d) Cross-sectional high-resolution TEM image taken at the boundary of the phase-change region in (c).(e) Inverse fast Fourier transform image of local areas from the active and inactive region in (d).
Cr-deficient composition and, thus, resulting in p-type conduction by Cr vacancies, as shown in Fig. S12 (upper figure).By lowering the working pressure during sputtering deposition, we obtained an O-free CrN film (designated as CrN') with a Cr/N composition ratio of 1.02.This N-deficient composition film with a NaCl-type cubic structure showed n-type semiconductor behavior, as shown in Fig. S12 (bottom figure).
this observation, the Si (111) and Si (311) settings of the double-crystal monochromators were used.1.3-μm-thick CrN films at N 2 flow rate of 6 SCCM were deposited onto both sides of aluminum foil for the measurements.A Si-N protection layer of 50 nm was in-situ deposited in the same sputter chamber to avoid the unwanted surface oxidation.The obtained EXAFS data were analyzed using Athena and Artemis software. 9Fourier transform (FT) of k 3 -weighted χ (k) spectra vs. radial distance (R) are shown in Fig.S13.Since that there is no other phase separated other than cubic-CrN phase in the film and the χ(k)-R spectrum was found to be deviated from the chromium oxide, we consider the first shell of theoretical cubic-CrN consisting of Cr bonded with 6 N atoms (R Cr-N = 2.08 Å) and 12 Cr atoms (R Cr-Cr = 2.95 Å).10The best fit was done for this plot within the k-range 3-16 Å and the fitting was performed in Rspace range of 1-2.8 Å. (Fig.S13) The corresponding fitting parameters are summarized in TableS1.In the CrN thin film, the fitted bond length is in a good agreement of theoretical ones, while the coordination number (CN) shows a relatively large deviation from the theoretical ones.The CN (= 5.41) of Cr-N only shows a slight decrease compared with 6.However, the CN (= 8.80) of Cr-Cr is far from the theoretical number of 12.This result confirms from the local bonding aspect that the CrN film shows a Cr deficiency non-stoichiometric property.

Fig. S13 .
Fig. S13.(a) Fourier transformed EXAFS data and fitting results of the CrN thin film; (b) Experimental and simulated Back-Fourier transformed EXAFS spectra.

Fig. S14 .
Fig. S14.The (a) transmittance, (b) reflectance and (c) absorption coefficient of CrN and CrN' thin film.The valance band maximum of (d) CrN and (e) CrN'

Fig. S15 .
Fig. S15.Simulated electrical field distribution.In (a) W plug with a 4-nm TiN adhesion layer and (b) TiN plug devices.

Fig. S16 .
Fig. S16.(a) Resistance as a function of the pulse voltage during the Reset process of a CrN memory cell-1; (b) R-T curve of CrN memory cell-1 in the HRS; (c) R-T curve of the CrN memory cell-2 in the HRS.

18 .
Fig. S18.Voltage and subsequent current flowing through the device (45 nm × 45 nm) when the Reset pulse is applied.

Table S1 .
Curve fitting results of Cr K-edge EXAFS in CrN thin films.

Table S2 .
Resistivity, thermal conductivity, density, and specific heat values used for the simulations.
Our results indicate that the resistance of both HRS and LRS strongly depends on the amplitude of the applied pulse voltage.Higher Reset voltage leads to a longer diffusion path of Cr and N atoms, resulting in a larger phase change volume and higher HRS resistance.Conversely, decreasing the applied Reset voltage decreases the phase change volume and HRS resistance simultaneously.When the Set voltage is further decreased, the high resistance CrN 2 volume cannot be fully Set back to CrN due to insufficient heat transformation, resulting in a higher resistance of LRS.Although larger Reset/Set voltage in our T-shape memory cell can enlarge the programing window, severe diffusion of Cr and N atoms under high voltage (Joule heating energy) makes it hard to self-heal after certain cycles of switching, thus accelerating the deterioration of the memory cell.Similar phenomena have been observed in traditional PCMs such as GST, which exhibits a strong Reset/Set energy dependence of endurance property due to unwanted atom migration in the T-shape device.The best cyclic property for both GST and GeTe is around 10 6~9 .